Do you have a custom recovery installed like Clockwork Mod or TWRP? If so you can do a factory reset in there. If not, to flash the former, download ROM Manager (you don't need the paid version); for the latter, GooManager.
Factory resets from these recoveries won't wipe your SD Card, so if you want to remove any media files, transfer those over manually and then delete. After a factory reset, you'll be prompted to enter in the new Google account and she'll be all set.
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